1. Field of the Invention
This invention relates to a light wavelength selection device for various optical systems such as a WDM (wavelength division multiplexing) optical communication system and a WDM optical CATV system. This invention also relates to a method related to such a light wavelength selection device.
2. Description of the Prior Art
WDM (wavelength division multiplexing) optical communication systems use a composite light beam having component light beams of different wavelengths respectively. Such optical communication systems are generally provided with light wavelength selection devices or optical demultiplexing circuits for separating a composite light beam into component light beams of different wavelengths respectively.
It is known to use a diffraction grating in a light wavelength selection device or an optical demultiplexing circuit. Generally, such a grating-based light wavelength selection device enables accurate tuning of light in a wide wavelength band.
In the grating-based light wavelength selection device, a composite light beam is applied to the diffraction grating, and is thus separated into component light beams of different wavelengths which travel along different directions from the diffraction grating respectively. Desired one of the component light beams is incident to an inlet of an optical fiber located at a given position relative to the diffraction grating. In this way, desired one of the component light beams is selected.
Regarding known grating-based light wavelength selection devices, it is generally difficult to easily adjust or change a selected wavelength (a tuned wavelength). The ability of easily changing the selected wavelength is effective and advantageous in making the devices usable in various systems.
Japanese published unexamined patent application 62-115403 discloses an optical tuner including a diffraction grating, a lens, an input optical fiber, an output optical fiber, a rotating mechanism, and a photodetector. In the optical tuner of Japanese application 62-115403, a composite input light beam having component light beams of different wavelengths is applied to the diffraction grating from the input optical fiber via the lens, and is thus separated into the component light beams of the different wavelengths respectively. The component light beams travel along different directions from the diffraction grating respectively, and then enter the lens so that they are focused on different points respectively. An inlet of the output optical fiber is located at one of the points of the focuses. Thus, one of the component light beams is incident to the output optical fiber. In this way, one of the component light beams is selected. The selected component light beam is guided via the output optical fiber to the photodetector. In the optical tuner of Japanese application 62-115403, the rotating mechanism serves to rotate the diffraction grating. This mechanism is activated to rotate the diffraction grating when the reception of another component light beam is required.
U.S. Pat. No. 4,815,820 discloses a method and an improved apparatus for aligning a diffraction grating, as, for example, one used in tuning the output of a laser. The total angular misalignment ".epsilon." between the incident and diffracted beams is a function of three angles: the tuning angle ".theta.", the angle of tilt ".beta." between the plane of the grating and the tuning rotational axis, and the angle ".alpha." between the grating lines and the projection of the tuning rotational axis In the plane of the grating. In the method and apparatus of U.S. Pat. No. 4,815,820, by adjusting only or only ".beta." to zero at two reference tuning angles ".theta..sub.O " and ".theta..sub.1 ", the residual alignment error Is made very small over the desired range of tuning angle ".theta.".